The ongoing miniaturization of components of a variety of devices makes high-resolution characterization of critical surfaces increasingly important. In the field of metrology, for example, surface-characterization devices such as stylus profilers and scanning probe microscopes (SPM) are routinely used to measure topography and other characteristics of critical samples. Stylus profilers and scanning probe microscopes are in fact frequently used as inspection tools to measure the critical surfaces of industrial devices like semiconductor chips and data storage devices during and after the manufacturing process. To be economically feasible, these profilers and scanning probe microscopes must complete their measurements as quickly, accurately, repeatably and as reliably as possible. The accuracy, precision, reproducibility, and reliability of such metrology instruments are especially critical in view of the ongoing desire that such surface-characterization instruments be capable of quickly and accurately characterizing dimensions smaller than those of the products and devices being fabricated, to assure manufacturing quality, and to provide accurate diagnoses of manufacturing problems. Because critical features continue to shrink in the manufacturing process, it is necessary to improve the accuracy and the speed of scanning probe microscopes and stylus profilers to keep up with the measurement demand.
For the sake of convenience, the discussion that follows and throughout this patent specification will focus on Atomic Force Microscopes (AFMs). In this regard, it shall be understood that problems addressed and solutions presented by the present invention shall also be applicable to problems experienced by other measurement instruments including surface-modification instruments and micro-actuated devices.
The typical AFM includes a probe which includes a flexible cantilever and a stylus mounted on the free end of the cantilever. The probe is mounted on a scanning stage that is typically mounted on a common support structure with the sample. A typical scanning stage may include an XY actuator assembly and a Z actuator, wherein “X” and “Y” represent what is typically the horizontal XY plane, and “Z” represents the vertical direction. “X” and “Y” and “Z” are mutually orthogonal directions. The XY actuator assembly drives the probe to move in an X-Y plane for scanning. The typical Z actuator mounted on the XY actuator and providing support for the probe, thus drives the probe to move along a Z axis which is disposed orthogonally relative to the X-Y plane. (The definition of the XYZ axes is convenient and typical, but the choice of axis name and orientation is of course arbitrary.)
AFMs can be operated in different sample-characterization modes including contact-mode and Tapping™ mode. In contact-mode, the cantilever stylus is placed in contact with the sample surface, cantilever deflection is monitored as the stylus is scanned over the sample surface, and the resulting image is a topographical map of the surface of the sample. In Tapping™ mode (a trademark of Veeco Instruments, Inc.) sample characterization, the cantilever is oscillated mechanically at or near its resonant frequency so the stylus repeatedly taps the sample surface or otherwise interacts with the sample. See, e.g., U.S. Pat. Nos. 5,266,801; 5,412,980; and 5,519,212 to Elings et al., which are illustrative.
In either sample-characterization mode, the interaction between the stylus and the sample surface induces a discernable effect on a probe-based operational parameter, such as the cantilever deflection oscillation amplitude, the phase or the frequency, all of which are detectable by a sensor. In this regard, the resultant sensor-generated signal is used as a feedback control signal for the Z actuator to maintain a designated probe operational parameter constant.
In contact-mode, the designated parameter may be cantilever deflection. In Tapping™ mode, the designated parameter may be oscillation amplitude, phase or frequency. The feedback signal also provides a measurement of the surface characteristic of interest. For example, in Tapping™ mode, the feedback signal may be used to maintain the amplitude of cantilever oscillation constant to measure the height of the sample surface or other sample characteristics.
In analyzing biological samples, polymers, photoresist, metals and insulators, thin films, silicon wafer surfaces, and other surfaces, the ability to accurately characterize a sample surface is often limited by the present ability of an AFM to move the stylus vertically relative to the surface at a rate sufficient to accurately measure the surface while scanning in either the X or Y direction. This ability is inadequate in present day devices for essentially two reasons.
In order to accurately measure the height of all features, both large and small, on a sample surface, the Z actuator must have the ability to displace the stylus connected thereto over a large range of heights, i.e., it must have large vertical travel. This necessitates that the Z actuator, whether it is a scanning tube such as is on this assignee's Dimension series AFM heads or is a flexure such as is on this assignee's Metrology series AFM heads, must be large enough to move the stylus up and down sufficiently to measure even the largest surface features.
Unfortunately, a necessary by-product of a larger Z actuator having greater range is associated greater mass which makes the actuator movement relatively slow. Slow actuators are not able to move the probe rapidly enough in Z while scanning in X or Y at anything more than modest speed without damaging the probe or sample or without sacrificing measurement accuracy. Because it is important while scanning to minimize the force of the stylus on the sample to prevent damage to the stylus and/or sample, the scan rate in X or Y must, of necessity, be reduced to a speed compatible with the Z actuator's ability to move the stylus up and over surface features without slamming into them, which is obviously undesirable. One present day technique to overcome this limitation and increase responsiveness of the Z-actuator is to increase the gain of its feedback loop. This works only to a limited degree because if the gain is increased more than a modest amount, the Z actuator begins to resonate and that resonance is passed into the AFM, creating parasitic oscillations, which in turn ruin image quality. In essence, a large mass, large displacement Z actuator cannot be made to overcome its inherent physical limitations.
In another approach, one does not attempt to wring more performance from the large Z actuator than it is inherently able to deliver. Instead, a separate “fast” Z actuator is used, with its own feedback loop, to move the stylus quickly over small surface variations that the large Z actuator is too slow to react to, which enables one to obtain relatively high quality imaging at even high scan speeds. The fast Z actuator is smaller than and hence of significantly smaller mass than the slow Z actuator. As a result, it is advantageously driven in its own (or shared) fast feedback loop at speeds exceeding that of the slow Z actuator.
Unfortunately, at high gain, the high speed of operation and momentum of the fast Z actuator can similarly cause parasitic oscillations which reduce image quality. A device and method which balances these inertial forces created by a fast Z actuator would be of great benefit and commercial interest.